The frequency of the eruptions appeared to have tapered off by Friday afternoon, however, although caution was still needed, a different official said.

"It appears that the volcano may be settling down slightly," he added. "The number of eruption-related earthquakes has decreased.

"But small to medium-scale eruptions could still continue, so we must remain vigilant," he said.

Television footage showed gray smoke mixed with ash billowing over the mountain last week afternoon.

"I've had experience with ash fall before, but never anything like this," a shopkeeper in the resort town of Karuizawa, at the foot of the volcano, told public broadcaster NHK television.

Earlier, the mountain was spewing smoke mixed with ash about 3,000 feet into the air, and monitoring cameras detected molten rock being thrown a distance of 600-1,000 feet from the summit during the pre-dawn hours last week.

ASH LIKE SNOW

Late on Thursday night, prevailing winds carried ash from the peak as far as central Tokyo. Television showed gray flakes falling over Shibuya, one of the city's liveliest entertainment districts.

Cars in Karuizawa were covered with gray ash. Enough ash had accumulated by morning that a dog being walked by its owner left prints on the street.

"It was really something. It was like snow," one young woman in Karuizawa told NHK. By 2 p.m. (1 a.m. EDT) some 572 earthquakes related to the volcano had been detected since the start of Friday, all of them too small to be felt by humans. Nearly 1,400 similar quakes were detected on Thursday.

Mount Asama, one of Japan's more active volcanoes, had its biggest eruption in 21 years on Sept. 1, spewing hot rock and raining ash on areas as far as 125 miles away.

"Another eruption on this scale is unlikely, but cannot be completely ruled out," the official added.

Senior members of Japan's Coordinating Committee for Prediction of Volcanic Eruptions said last Thursday there was no data suggesting that a large-scale eruption of Mount Asama was imminent.

But the panel - made up of government officials, disaster prevention experts and academics - said the possibility of repeated "explosive eruptions" similar to those that occurred on Sept. 1 could not be ruled out, and that the volcanic activity needed to be monitored carefully.

Mount Asama has had several minor eruptions in recent years, including at least four in 2003, but the one on Sept. 1 was the biggest since April 1983.

Japan, located in the so-called Ring of Fire, an arc of volcanoes and oceanic trenches partly encircling the Pacific Basin, has 108 active volcanoes, according to the Meteorological Agency - some 10 percent of the world total.

In 1991, 43 people including police, fire officials and journalists were killed by a lethal mix of steam, ash and rock when a volcano erupted on the southern island of Kyushu.